1. Field of the Invention:
The present invention relates to methods of making semiconductor devices, more particularly to methods of making semiconductor devices by the adoption of an improved set of selective oxidation steps using anti-oxidation films as oxidation masks.
2. Prior Art:
It has become widely known, as LOCOS (localized oxidation of silicon) or ISOPLANAR structures, to selectively oxidize silicon substrates by use of silicon nitride films as anti-oxidation masks. But, as a result of strong demand of high integration degrees in ICs (integrated circuits), high resolution in IC pattern forming is more and more demanded. Consequently, it is revealed that the conventional pattern forming is insufficient, that is, it is difficult or impossible to obtain high resolution IC patterns with shapes precisely same as those of anti-oxidation films employed as a mask, for the following reasons to be described.
The conventional processes will be described by referring to the accompanying drawings. FIG. 1(a) and FIG. 1(b) are cross-sectional views of a semiconductor structure at process steps to obtain a conventional LOCOS structure. A silicon nitride (Si.sub.3 N.sub.4) film 2 is deposited by a known CVD (chemical vapor deposition) method on the entire face of a front surface of a silicon substrate 1. In this case, a thin silicon oxide layer (not shown) may be formed on the silicon substrate 1 before the deposition of the silicon nitride film 2, in order to reduce strain produced at the interface between the semiconductor substrate 1 and the silicon nitride film 2, if the latter 2 is formed directly on the former 1. A photo-sensitive material is applied onto the silicon nitride film 2, followed by light exposuring to form a photo resist film pattern 3 for selective oxidation, by using a photomask. Then, the silicon nitride film 2 is etched by use of the photo resist film pattern 3 as an etching mask thereby copying the photo resist film pattern 3 onto the silicon nitride film and obtaining a silicon nitride film pattern 2 as shown in FIG. 1(a).
Thereafter, the photo resist film pattern 3 is removed, and the silicon substrate 1 is heated in an oxygen atmosphere in order to grow an oxidation film 4. The oxidation film 4 is formed at the surface of the silicon substrate 1 not covered with the silicon nitride film pattern 2, as a result of the oxidation of the silicon substrate 1. During the oxidation the oxygen atoms can not diffuse into the silicon nitride film pattern 2, which thus serves as an anti-oxidation film. Removing of the silicon nitride film pattern 2 by use of hot phosphoric acid heated at about 150.degree. C. is a final step of the selective oxidation process as shown by FIG. 1(b).
The selective oxidation process described above has been employed to selectively oxidize the surface of the silicon substrate 1 by use of the silicon nitride film 2 as the mask for the selective oxidation, and to form separated insulating regions on and/or in the semiconductor devices. But such a conventional selective oxidation process has the following shortcomings. FIG. 1(c) is an enlarged cross-sectional view showing the right half-portion of the silicon substrate 1 shown in FIG. 1(b). The silicon substrate 1 has a structure shown in FIG. 1(c), when investigated microscopically. That means, the oxidation film 4 is not uniformly formed on the surface of the silicon substrate 1. A brim portion 41 of the oxidation film 4 grows under an end portion of the silicon nitride film pattern 2 thereby to raise an end portion 21 of the silicon nitride film 2. The brim portion 41 of the anti-oxidation film 4 is so-called "bird beak" region. In addition, the oxidation film 4 has an upheaved protuberance 42 called as "bird head". The formation of these irregular oxidation film portions 41 and 42 arises from the fact that the oxidation atoms uniformly diffuse into the silicon substrate 1 underneath the end portion of the silicon nitride film, and that after the oxidation the volume of the silicon substrate surface swells twice as large as the initial volume thereof. The abovementioned irregular oxidation spoils uniformity of the resultant oxidation film and reproducibility of the oxidation film pattern. Thus, it is inevitably necessary to suppress the irregular oxidation as much as possible, when fine and accurate oxidation film patterns are required to be formed with a superior controllability.
Another conventional selective oxidation process will be described by referring to FIGS. 2(a) to 2(c). FIGS. 2(a) to 2(c) are cross-sectional views showing semiconductor device structures at several processing steps. In this case, the exemplified method purports to diminish uneven surface areas on the surface of the semiconductor devices. Such semiconductor devices are produced by the following steps. A silicon nitride film 2 is formed on a silicon substrate 1, and a photo resist film 3 is applied on the silicon nitride film. Then, a photo resist film pattern 3 is obtained by using a photo mask with a specified pattern, and the silicon nitride film 2 is selectively etched by use of the photo resist film pattern 3 as an etching mask thereby forming a silicon nitride film pattern 2 as shown in FIG. 2(a).
By further continuing the etching for the front surface of the silicon substrate 1, concave portions 11 are formed as shown in FIG. 2(b). A depth of the concave portions 11 formed in the silicon substrate 1 is selected to be as thick as about a half of a desired thickness of an oxidation film to be formed later. Next, the photo resist film pattern 3 is removed, and selective oxidation is carried out by use of the silicon nitride film pattern 2 as an anti-oxidation mask. The silicon nitride film pattern 2 is stripped off thereafter as shown in FIG. 2(c).
The semiconductor device structure shown in FIG. 2(c) has a much more even principal surface as compared with the case of FIG. 1(b). But, there still exist bird beak regions 41 and bird head regions 42 in the resultant oxidation film 4 as shown in FIG. 2(c). Although the selective oxidation process is widely employed in the semiconductor device industry, it has the shortcomings that the obtained selective oxidation films inherently have irregularly oxidized regions known as bird beak and bird head, as described above. In addition to the case of oxidizing the single crystalline silicon substrate, similar problems are encountered in selective oxidation of polycrystalline silicon films. Such selective oxidation of poly-Si films is necessary, for example in forming poly-Si conductive patterns or poly-Si gate regions for MOS-type field effect transistors.